For the large screen TVs exceeding 40 inches, projection TVs are in more widespread use than direct-view Braun tubes. The projection TV projects an image of a Braun tube (PRT) having a nearly 5.5-inch screen onto a screen of about 40 inches by use of an optical lens, mirror, etc. In general, a color image is obtainable by projecting, on a screen, images of three Braun tubes respectively in red, green and blue colors.
For this purpose, a Braun tube called a projection tube (PRT) is used. In the projection type display, a PRT image, for example, of approximately 5.5 inches is projected onto a 40-inch screen, resulting in image magnification of an area as large as 50 times. Due to this, the PRT image is required to be extremely high in brightness and of good focusing quality. For realizing high brightness, a large electron beam current is needed.
The PRT however possesses a problem in that a favorable focusing characteristic must be maintained even when a large current is flowing. Consequently, the PRT generally uses a so-called Hi-UPF electron gun that has a comparatively good focusing characteristic even in a large current region. U.S. Pat. No. 4,178,532 discloses an example of a Hi-UPF type electron gun.
For the PRT, attention has conventionally been paid, principally, to an increase of the main lens aperture. This is for the purpose of maintaining a focusing characteristic even with a large current. In an attempt to further improve performance in the large current region, there is a proposal of a so-called large-aperture electron gun having a main lens with an increased aperture. U.S. Pat. No. 4,271,374 discloses such an electron gun as an example.
Except for a conventional main lens, U.S. Pat. No. 4,318,027 discloses a means to improve focusing in the structure of a pre-focus system. This however concerns use of a BPF electron gun that is different in electron gun structure. Furthermore, as a method of improving focusing, it is possible to increase the size of the electron gun to thereby increase the main lens aperture. This however will require an increase also in the diameter of the tube neck, resulting in side effects, including an increase in the electric power needed for deflection, etc.
According to the present invention, a focusing characteristic can be improved, which is equivalent to or greater than an increase in the main lens diameter without increasing the tube neck diameter.
A first characteristic of the present invention is, in an Hi-UPF type electron gun, to improve the focusing characteristic in large and small current regions by making a control grid (G1) hole diameter equal to or smaller than xcfx860.57 mm and the distance between an acceleration electrode (G2) and a first anode electrode (G3) equal to or smaller than 1.9 mm.
A second characteristic of the invention is to particularly improve focusing in the large current region by making the G3 hole diameter on a G2 side xcfx86mm or smaller.
A third characteristic of the invention is to improve the focusing characteristic in the small current region by making the G2 plate thickness 0.37 mm or smaller.
A fourth characteristic of the invention is to obtain an excellent life characteristic by using an oxide containing barium scandate in the cathode.
A fifth characteristic of the invention is to further improve focusing performance drastically by combining the above pre-focus-structured lens structure with a large diameter lens.